A paper has been accepted for publication in a science journal (PDF) where the author has analyzed data from NASA’s Kepler planet-finding observatory, trying to figure out how many Earth-sized planets there might be in the galaxy orbiting their stars in their habitable zones; that is, at the right distance so that the star warms the planet enough to have liquid water. In the paper, he estimates that on average 34% (+/-14%) of Sun-like stars have terrestrial planets in that Goldilocks zone.

34%!

I can explain how he got this number. But I can also explain why I think this needs to be taken with a grain of salt. Let me be clear: it’s possible he’s right, and I suspect he may very well be. His math looks good to me. But a couple of assumptions he had to make need to be pointed out, and I want that to be clear before the media start running around saying there are billions of Earths in the galaxy based on this.

Here’s the deal. Kepler is an orbiting observatory that’s staring at about 100,000 stars, looking for dips in their light when an orbiting planet passes in front of them from our perspective. The length of time the dip takes gives us the orbital period of the planet, and the size of the planet (if the star’s size is known, generally true) can be determined by how much light is blocked. I talk about how this works in a little more detail in an earlier post.

The astronomer, Wesley Traub of Caltech, based his analysis on only the first few months (136 days) of Kepler data, what was available at the time. This introduces a bias into the calculations, because that length of time is too short to conclusively find planets in their stars’ habitable zones! Even being generous, the length of such an orbit is at least 200 days, much longer than the Kepler sample. So he was forced to look at only short-period planets (with periods of 42 days or less), much closer to their stars, and extrapolate the data from there. I’ll note that Dr. Traub was up front about potential biases in the data and his analysis.

He looked at stars similar to the Sun (with a range from somewhat hotter to somewhat cooler, roughly F, G, and K stars). He then looked at data for all planets detected — terrestrials (Earth-sized), ice giants (like Uranus and Neptune), and gas giants (like Jupiter), getting their size and orbital period.

Then he found the ratio of terrestrial planets to all the planets seen. Again, remember, this ratio was found for planets somewhat close in to their stars.

Then he plotted all the planets versus distance from their parent stars. For example, you see very few planets very close to the star (probably because it’s hard to form or get a planet to orbit that close in), then more as you get farther out, then fewer at some large distance (which may, once again, simply be due to the fact that planets with long orbital periods can’t be seen in the short duration of the data). He then found an equation (called a mathematical fit) that did a good job predicting the shape of the plot. Once he had that, it’s easy enough to extrapolate it out to the distance of the habitable zones of the stars.

That gave him an estimate of all planets orbiting there, including gas and ice giants. Multiply by the ratio of terrestrial planets, and boom! 34% of stars like the Sun should have planets that are Earth-sized orbiting them at the right distance.

The thing that makes me most uncomfortable is that he had to use those short period planets, and extrapolate outwards. Extrapolation is always dangerous because you can’t be sure your fit behaves well outside the range in which you calculated it. For example, imagine you took a census of 1000 people ages 0 – 17, and made a fit to their height versus age. You’d find their height gets bigger with time, in general. But if you extrapolate that out to someone who is 40 years old, you might estimate they’ll be 4 meters tall!

We don’t know very well how planets form in their solar systems, and how they move around after. It may be that nature doesn’t make many planets in the habitable zone. Or maybe it does, but after some amount of time the planets move out of it, maybe through gravitational interactions with other planets. I’ll note that our own solar system makes that seem unlikely; we do have three planets in the Sun’s HZ!

So what are we to make of all this? I think Dr. Traub did careful, interesting work, and his number of 34% is probably not terribly far off. Again, we should keep our eyes on that number, since it’s based on extrapolation, but the calculation that went into it is well-reasoned. I wouldn’t be surprised if he’s pretty close to the mark.

And what does this mean for you, the science enthusiast? Well, F, G, and K stars comprise very roughly a quarter of the stars in the Milky Way, or something like 50 billion stars total (again, I’m being really rough here). That means, assuming Traub is correct, there could be 15 billion warm terrestrial planets in our galaxy alone!

I’ll add that I think this work was worth doing even this early on in the Kepler mission. This is a great first step in analyzing the massive amount of Kepler data, and putting a number on it that we really want to know. As time goes on, and Kepler sends back more observations, Traub’s work will have paved the way to work on planets with bigger orbits. I’ll be very, very curious to see which way that number moves as more data come in.

The thing that can be said for certain about all this is that the universe will surprise us when we actually do find out more.

And this is only for F, G, and K type stars. The permutations with other types of stars and planets will surely surprise us as well (i.e. like Deen’s question). Now if we could only get more funding to explore these questions. (Maybe if we tell Congress there is oil on them?)

What I like about panspermia theories is that it gives life a lot more than 4 billion years to develop. Having all those liquid water planets would seem to make it more likely that water-based life would develop and spread around (or at least their molecules).

It would make our water planet less special and make more sense from an evolutionary perspective (at least to me).

In the last few hundred years, we go from being the center of the universe to just another watery planet.

It seems to me like any decent guess based on this method would likely underestimate the total. Like Deen mentioned, there could be moons, and Kepler has to be sitting pretty much right on the plane of the planets’ orbits to even see the transit, right? Any solar systems with an inclined orbit from our perspective wouldn’t be seen other than planets close to the star.

You should also mention that a planet “in the habitable zone” doesn’t necessarily mean a planet that anything could live on. Venus and Mars form the edges of the habitable zone in our system, and neither is habitable by any kind of life that we know of, let alone complex multicellular life.

Can’t wait for more solid data from Kepler to be cracked. Ideally it would be something like 1400 days – enough for planets at distance of 1AU from their stars to transit 3 times in front of their yellow stars, to give certain finds

Then we’ll have pretty solid estimates for whole galaxy (and very very solid for red dwarf habitable zones).

Sorry if this is a bit off-topic, not to mention obscure, but that Dan Durda painting of the watery moon in front of the Juptier-like planet reminds me very much of the opening credits of the 1970’s TV show Space: 1999. About midway through the opening theme/credits, a view almost exactly like that–only flopped (reversed) and with different colors–appears. Well, that and the moon in the show has no atmosphere. And the moon’s behind the planet…
OK, FINE, it’s less exact than I thought. But it’s very similar. (I challenge any Space: 1999 fan to see the painting, and not make the connection!) Judge for yourself: http://space1999.net/catacombs/main/images/space/titles/spty1023b.jpg

Despite all the Bad Astronomy, I dug that show. I own season 1 on Blu Ray. Some friends and I once came up with a way to fix a lot of the issues and still have the same basic scenario. Yeah, it was made up fictional science (a test of an inertialess space drive on the Moon works way, way, way better than anyone expected), but it was at least internally consistent.

We may never reach any of these planets, but at the very least it provides some hard science for science fiction. We knows these planets are actually out there now. Before these discoveries, the very existence of any other habitable worlds was only theoretical.

I think we’re spending too much time and energy on earth-like planets in the so called habitable zone. This sort of analysis completely neglects gas planet/moon systems like Jupiter/Europa where liquid water oceans appear to exist under thick layers of water ice, kept liquid by the internal friction within the moon due to the gravitational force of the planet. Europa and Saturn’s moon Enceladus appear more likely candidates for life then Mars and certainly Venus.

Interestingly, Torbjörn Larsson, a Swedish observer, noted on the Panda’s Thumb site that a recent claim that there may be hundreds of billions of rogue planets wandering around the Milky Way Galaxy (and presumably other galaxies) could include Jupiter/Europe systems that could also support life (e.g. in such a system, there is no necessity of a central star). This would greatly increase the number of planets/moons that could support life.

The search for the goldilocks zone planets is classic ego-centric human thinking.

We’ve gone from:
My cave is the centre of the universe. to
My nation is the centre of the universe. to
Earth is the centre.
The sun is the centre- to a new breed:

“My life-form is the centre of all life”.

Whereas, I understand that we look for life similar to us, because we understand what it takes- and we know life can exist in planets such as ours- and we hope panspermia is real.

However, I can’t help but believe we will find that our code- DNA is not the centre of all life. Water may not be a requirement.

If there is life out there we could well turn out to be the exception not the norm as to how life is. There could be lots of life out there, we just don’t know where/how to look for it- because we’re busy trying to find our mirror image.

The reason we’re looking for life like us is because we have proof that it works (we’re here, aren’t we?). We’ve only just reached the point in our history where we’re technologically capable of searching for life in the galaxy. We might as well start with something we know how to look for. If that doesn’t work, we can then move on to unimaginable forms of life that are beyond our experience.

Why mention the PDF (Planetary Defense Force) in this article? Are we afraid there are some evil ETs living on these planets that have designs on taking over our planet? Just like in (almost) every movie involving aliens.

@20 – Bean Soup
The seach for the goldilocks zone is NOT ego-centric; it’s based on some rather objective observations. If anything, what’s ego-centric is armchair quarterbacking the goldilocks zone based on some sci-fi fantasy about exotic life forms. The assumption that life needs at least liquid water and an energy gradient isn’t based on the fact that we’re a bunch of watery meat-bags living cushy lives in the temperate zone; it’s based on expeditions to extreme environments. The planet Earth itself has all kinds of inhospitable areas, and they’re BARREN. It’s not like there’s much competition from us meat-bags for the lush resources at the dry, radiation-bombarded ice of Vostok Station or the glowing-hot magma of Kīlauea. If there’s some hypothetical extremophile that needs no liquid water to live, we’ve been to places where they should thrive. There’s nothing. Maybe some dormant microbes waiting to thaw out if it takes millions of years. On the flip side, there are other seemingly inhospitable areas teeming with life — toxic lakes, subterranean lakes, deep-sea trenches, black smokers, oxygen-deprived lakes. What do they all have in common? Energy gradient and. . . liquid water. In fact, black smokers (no sunlight but plenty of water) are much more biologically active than open deserts (lots of sunlight, little water). Is this ego-centric, or just a rational observation?

Absent evidence to the contrary, scientists will look for life where there’s liquid water. Anyone is free to journey to the Pole of Cold or the depths of a volcano to prove them wrong, but they’d better bring back a sample.

one thing always bothers me about calculations of a star’s HZ or “goldilocks zone”:

can we even say that the earth is in the sun’s HZ?

from what i can tell, this is not an idel question. the presence of liquid water on the surface seems far more dependent on atmospheric conditions than on distance to the star. then again, i suppose that would just make the true HZ bigger than current estimates.

A fussy point about the calc of Phil’s 15 billion warm terrestrial planets. 34% of 50 billion stars gives 15 billion stars with warm terrestrial planets. That number would need to be multiplied by some number greater than one for the average number of planets in the zone per star of that 15 billion that has at least one.

As Phil points out, careful calcs are valuable even with uncertain numbers.

I really am confident when the more firm data is available in a year or two, it will bear out Dr. Traub’s preliminary analysis. And as for the media, they will distort things no matter what, because they always need “news at 11″ or today they call it the “news cycle”, and I call it make believe work, or why do they need all these reporters?

Interesting, but totally premature. There’s no hard data to support it and nothing that critically depends on it. When we actually HAVE a decent sample of terrestrial planets, then we can draw conclusions.

I think any response to this research I have may be biased, by the simple fact that my first response to the “34%” conclusion was “please oh please let him be right”. I’ll have a closer look at it when I’m feeling less tired and more cynical…

There are so many definitions of the habitable zone it is difficult to tell what is meant.

Certain extremely liberal definitions would include Venus and Mars. More conservative ones only include the Earth and the Moon (hey it is planet size, the Stern-Levison criterion suggests it would count as a planet if it were in independent solar orbit, and it is unequivocally in the HZ…) Certainly several of the more recent ones suggest that Venus lies closer to the Sun than the inner HZ boundary (but since the zero-age main sequence Sun was fainter than the current Sun, it may have been in the HZ in the past), but that an Earth or super-Earth in Mars orbit could be habitable.

And the assumption that the size distribution is independent of orbital period is highly suspicious: radial velocity surveys indicate that there are two major populations of giant planets: the hot Jupiters with periods of less than ~10 days, and the eccentric giants at periods of ~100 days or more, with very few giant planets at intermediate periods…

Let me be a pessimist. Let’s say the 34% is way off and its really only 5%. That’s still 2.5 billion HZ planets in the Milky Way. I”m still blown away!! Seriously, take a deep breath and think about them apples.

Well, F, G, and K stars comprise very roughly a quarter of the stars in the Milky Way,

Really?

Main-sequence~wise type M stars – red dwarfs make up 80% of all stars I thought. 10% odf stars being white dwarf stars and less than 1% O-B stars and supergiants and giants. K type orange dwarfs are about 15% of stars. F & G type were about 5% approximately of stars if I gather right – and that’s excluding the L & T brown dwrafs as sub-stellar objects into the bargain.

So say, 5% of stars G & F yellow and Procyonese dwarfs plus 15% of orange dwarfs like Epsilon Eridani gives us 20% – and note the hotter end of class F (down to say F5 or so) and cooler end of class K (below say K5 or so) may well not be suitable for life.

There’s still so much left to learn and understand before we can really say with any confidence on this bit its certainly intriguing and marvellous to get some preliminary assessments even if they do come with an awful lot of question marks.

.. we do have three planets in the Sun’s HZ!

Eh? We do? Are you counting Earth’s Moon as a planet there or something?

Earth and Mars are in the Solar Habitable Zone – Mars suffering because its too small in mass to sustain plate tectonics and a magnetic field and hence cannot retain enough of an atmosphere if I understadn correctly (& okay maybe Idon’t) but Venus is -at least currently – too far towards our daytime star’s Too Hot zone as I understand it.

Or does Ceres just squeak into the HZ and count? Dwarf planets after all are planets too. Would’ve thought it was too far into the Too Cold zone but maybe not.

The Habitable Zone, of course, changes over time as the Sun (or other stars) brighten with age and then there’s the whole question of possibly habitable worlds outside the HZ such as Europa, Enceladus and Titan to consider.

So why Drake’s Paradox? Because conservatives on those worlds will eventually shut down any space-based operations not oriented towards nuking their enemies or monitoring their citizens, as they cost too much and don’t tend to promote religion. Expect a lot of worlds armed to the teeth just waiting for the unsuspecting alien explorers to dare to show their faces, if they haven’t already bombed themselves back to the stone age (or whatever they had). The handful of more enlightened (and less paranoid) liberal worlds that survive might be inclined to leave other worlds alone out of more positive motivations. Probably a good thing that interstellar travel isn’t easy. In a different universe, where it is, history might look like what happened here on earth, with Hawking radiation used as a weapon.

.. history might look like what happened here on earth, with Hawking radiation used as a weapon.

Que?!?

When have we used Hawking radiation as a weapon ever? We haven’t even created any black holes as yet to generate Hawking radiation & have only relatively recently discovered it!

(Can’t quite see how Hawking radiation would work as a weapon anyhow. Click on my name for Hawking radiation wiki-page.)

Also throwing Earthly (USA parochial even) political references into this thread? Really?

Human politics are unintelligent and dumb enough. Do we honestly think that alien sentiences more advanced than our own and applying to quite probabaly possibly different lifeforms will really be a good match for our politics? Can’t imagine what alien politics & governance systems would be like. (Or can have a few wild guess’es but that’s about all.)

Main-sequence~wise type M stars – red dwarfs make up 80% of all stars I thought. 10% of stars being white dwarf stars and less than 1% O-B stars and supergiants and giants. K type orange dwarfs are about 15% of stars. F & G type were about 5% approximately of stars if I gather right – and that’s excluding the L & T brown dwarfs as sub-stellar objects into the bargain.

Messier Tidy @43 — I agree with you about using our politics for other critters on other planets. But, if we have a billion or so to talk about, it seems nearly impossible that at least a few of them wouldn’t be aggressive in some way.

However, I can’t help but believe we will find that our code- DNA is not the centre of all life. Water may not be a requirement.

If there is life out there we could well turn out to be the exception not the norm as to how life is. There could be lots of life out there, we just don’t know where/how to look for it- because we’re busy trying to find our mirror image.

OK, this is plausible, as far as it goes. What do you suggest we do about it? Stop looking for life that is like Earth life and start thinking about ways to detect life that is inconceivable to us? How should we go about this?

The problem with life that is not as we know it is simply that – we don’t know how to recognise it. We still have trouble coming up with a definition of “life” that actually applies to all the life we do know about here on Earth.

In the meantime, I’m quite happy that folks are seeking out new life elsewhere, on the basis that life out there might share some fundamental parameters with life here.

The assumption that life needs at least liquid water and an energy gradient isn’t based on the fact that we’re a bunch of watery meat-bags living cushy lives in the temperate zone; it’s based on expeditions to extreme environments. The planet Earth itself has all kinds of inhospitable areas, and they’re BARREN

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That’s not quite true though. You can go anywhere from sub-zero frost to boiling sulphur springs and find life. There is even life kilometres below the earth’s surface.

There is bacteria found living (and mutating) on the external surfaces of our satellites.

Look at the humble waterbear that can survive all sorts of extremes- although granted needs more temperate environment to thrive and reproduce.

It may be that life on our planet prefers the cushy temperate zones- but life that evolved from different chemistries would not have the same requirements.

How to look for life that is different to ours when we don’t know what it will be? Look for the “un-natural”- look everywhere. Look to see what seems to violate the norms. There is most likely life out there- but it almost certainly doesn’t look like us… and no matter what the sci-fi lovers would like to believe…

About the best definition I’ve heard for life is “any pattern of mass/energy that is both self sustaining and self replicating”. That pretty much covers anything from solid state electronics to plasmas.

Yes it is. The post you were responding to was describing the whole earth, not just the surface. There is no life in the mantle, no life in the core. Even on the surface there are large swaths of Antartica that are sterile, and regions in the Atacamba desert that are sterile.

You can go anywhere from sub-zero frost to boiling sulphur springs and find life. There is even life kilometres below the earth’s surface.

All these examples still constitute only a thin fracture of the surface crust of the earth. As extreme as they are from our human perspective, even from the perspective of the rest of the earth, they are not extreme at all.

It may be that life on our planet prefers the cushy temperate zones- but life that evolved from different chemistries would not have the same requirements.

But that begs the question, if it really is so easy for life to appear with such different chemistries, as to why life didn’t evolve with these different chemistries here on earth in those zones with those different requirements. Certainly it is not the life-as-we-know-it outcompeting them, since life-as-we-know-it does not and cannot survive in those parts of the earth.

And to reiterate, when you contemplate a voyage into the unknown, it is true that you should always keep an open mind about your destination. However, you must still embark from a familiar port. There is no other way to do it.

The B.A. left out all kinds of interesting things about this. For one, a paper by Catanzarite and Shao has already been published that takes the same Kepler data and comes to a dramatically different conclusion. Traub references that paper and does a direct comparison between his result and that of Catanzarite and Shao. They differ by about a factor of 30x(!) which seems to be well accounted for by the different assumptions. So, we can say with reasonable certainty that eta-earth is somewhere between about 1% and 35%. The difference lies in how incomplete the Kepler data is (so far) on periods longer than 42 days. It should be noted that the Kepler team DID announce several planets, some of terrestrial size and in the habitable zones of their stars, with periods LONGER (sometimes by almost a factor of 2), than 42 days, so clearly the team has looked ahead into their data that hasn’t been publicly released (it would be stupid to think that they haven’t). So, the big difference between these papers lies in how complete one thinks their look-ahead has been. Traub was strict about his cut-off; CS perhaps more realistic if less statistically “sound”. One shouldn’t hold out a lot of hope that true solar analogs with periods near 1 year around G stars will change these numbers very much as there are just too few such stars and we already know that a good fraction of them have solar systems completely unlike our own.

That’s not quite true though. You can go anywhere from sub-zero frost to boiling sulphur springs and find life. There is even life kilometres below the earth’s surface.

This is true, but you find less and less as you get away from the “water available in the range of 0 – 40 °C” zones.

Yes, there are bacteria that live in antarctic glaciers, but they have to excrete proteins that melt the ice in which they live (just a tiny bit) so they can absorb nutrients and grow. Yes, there are thermophilic bacteria, but the higher the temperature of the spring, the fewer species you will find.

There is bacteria found living (and mutating) on the external surfaces of our satellites.

Citation needed.

Look at the humble waterbear that can survive all sorts of extremes- although granted needs more temperate environment to thrive and reproduce.

And, more imortantly, to metabolise. They survive extreme conditions (radiation, vacuum, dessication (sp?)) by going dormant. It’s hard enough to identify life from a distance when that life is active, but to detect dormant life from light-years away would be very much more challenging.

It may be that life on our planet prefers the cushy temperate zones- but life that evolved from different chemistries would not have the same requirements.

Absolutely correct, and completely useless. Without knowing something about this other life, how could we ever begin to come up with a way of detecting it from a distance?

What has me interested is David Brin’s suggestion (though he probably didn’t come up with it) that the most probable solution to Fermi’s Paradox is that while earth-like worlds are common around sunlike stars, they’re very likely to be water worlds. That is, entirely covered with water, not just a mix of continents and oceans like Earth.

When you look at how thin the oceans are, it’s amazing that we aren’t completely water covered. And if that’s the case, you can easily have tons of life, even advanced life…even intelligent life. But technological life?